Monk was at a loss to explain his side's poor performance after he had stuck with the same starting XI that thumped Norwich 3-0 last weekend.
"I thought it was poor defending in the build-up to the goal and then Angel has to do much better at the back post," he said.
"You have to be stronger than that. You have to be willing to get hurt.
"That's what you have to do at the back post - you know it's coming. He knew that cross was coming, he had time to think. It's not like it's a split-second.
"We have been working all week on those sorts of situations, so that's the disappointing part for myself."
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